1. The Field of the Invention
The present invention relates generally to systems and devices that include at least one component rotatably supported by a bearing assembly. More particularly, embodiments of the present invention relate to bearing assemblies that include various features directed to controlling ingress and egress of foreign matter to and from, respectively, the bearing assembly, and which thereby materially reduce the incidence of foreign matter related problems in the bearing assembly, as well as in the system or device wherein the bearing assembly is employed.
2. Related Technology
X-ray producing devices are valuable tools that are used in a wide variety of industrial, medical, and other applications. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis and testing. While they are used in various different applications, the different x-ray devices share the same underlying operational principles. In general, x-rays, or x-ray radiation, are produced when electrons are produced, accelerated, and then impinged upon a material of a particular composition.
Typically, these processes are carried out within a vacuum enclosure. Disposed within the vacuum enclosure is an electron source, or cathode, and a target anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes a stream of electrons to be emitted by the process of thermionic emission. A high voltage potential applied across the anode and the cathode causes the electrons emitted from the cathode to rapidly accelerate towards a target surface, or focal track, positioned on the anode.
The accelerating electrons in the stream strike the target surface, typically a refractory metal having a high atomic number, at a high velocity and a portion of the kinetic energy of the striking electron stream is converted to electromagnetic waves of very high frequency, or x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray tube for penetration into an object, such as the body of a patient. As is well known, the x-rays can be used for therapeutic treatment, or for x-ray medical diagnostic examination or material analysis procedures.
The aforementioned scheme for the production of x-rays is relatively inefficient. It is generally acknowledged that in typical x-ray tube operations, only about one percent of the energy contained in the beam of electrons produced by the electron source results in x-ray emissions from the target surface. A substantial portion of the remaining energy of the electron beam is imparted to the x-ray device and its component structures, such as the anode, in the form of heat. In general, the quality and resolution of images produced by an x-ray device increases in relation to, among other things, the power associated with the electron beam. Thus, improvements in image quality have often come at the cost of a relative increase in x-ray tube operating temperatures. Various approaches have been devised to deal with such increases in operating temperatures.
X-ray tubes that employ rotating anodes represent one approach that has been successful in managing the high heat levels characteristic of many x-ray devices. In a typical rotating anode type x-ray tube, the anode is rotatably supported by a bearing assembly. A stator serves to rotate the shaft, and the anode accordingly rotates as well. As the anode rotates, each point on the focal track is rotated into and out of the path of the electron beam generated by the cathode. In this way, the electron beam is in contact with a given point on the focal track for only short periods of time, thereby allowing the remaining portion of the focal track to cool during the time that it takes such given portion to rotate back into the path of the electron beam.
As suggested above, the bearing assembly with which the shaft is rotatably supported is central to the operation of such rotating anode type x-ray devices. However, many known bearing assemblies and associated components present problems which often act to materially impair the safety, effectiveness and reliability of the x-ray device. In particular, the design, assembly, and operation of typical bearing assemblies are such that many known bearing assemblies often become contaminated by particles which create various problems with respect to the operation of the x-ray tube.
One example of problems caused by the presence of particles in the x-ray tube relates to the high voltage across the cathode and anode. In general, the presence of such particles in the x-ray tube causes the high potential across the cathode and anode to become unstable. This lack of high voltage stability causes discharges of electricity, or arcs, between the cathode and anode. High voltage arcs may cause damage to the target surface of the anode, as well as to the cathode. Further, such high voltage arcs may compromise image quality through the generation of x-ray image artifacts.
Another problem associated with the presence of particles in the x-ray tube relates to the operation of the bearing assembly. In particular, when such particles enter, or are created in, the bearings, the particles tend to stick between the balls, creating rough surfaces inside the bearing. As the balls pass over the rough surfaces thus created, noise is generated within the x-ray device. Such noise is distracting to the operator. Further, such noise can be unsettling to a patient, particularly in applications such as mammography where the patient is in intimate contact with the x-ray machine. Finally, the rough surfaces created by the particles may serve to reduce the operating life of the bearings.
There are a variety of mechanisms by which foreign matter contamination of the x-ray tube may occur. As suggested above, at least some of such mechanisms concern the design, assembly, and operation of bearing assemblies. For example, many known bearing assemblies employ balls and/or races which are designed to include a solid lubricant such as silver (Ag), lead (Pb), or molybdenum disulfide (MoS2). Generally, such metal lubricants are employed in x-ray devices at least because they are better suited to the extreme operating temperatures of an x-ray tube than are typical hydrocarbon-based lubricants. These solid lubricants are applied to the balls and/or races by various chemical or physical methods such as electroplating, ion plating, sputtering, and evaporation. While the coating process is controlled in an effort to ensure uniformity and adherence of the coating, the motion of the balls rolling along the races causes the metal lubricant to move around somewhat and form lumps. Such lumps of lubricant are often as large as a few thousandths of an inch thick, and may be as long as one eighth of an inch.
As the lumps of lubricant are formed in the bearing assembly, they additionally pick up various elements, such as iron (Fe) and nickel (Ni), from the balls and races of the bearing assembly. As a result of the presence of the iron, the lumps of lubricant typically exhibit magnetic properties. Typically, these other elements migrate to the lubricant as a result of processes such as solid state diffusion, and abrasion. As the lubricant becomes contaminated with such elements, the desirable properties of the lubricant are compromised. For example, the ability of the lubricant to adhere to the balls and races is impaired, and the lubricant thus tends to separate from the balls and races and is then able to move about within the x-ray tube. As discussed above, problems caused by the loose lubricant particles include arcing between the cathode and anode, creation of noise in the bearing assembly, and a shortening of the life of the bearing assembly and its components.
Yet another vehicle by which foreign matter contaminates the x-ray tube relates to the processes by which the bearing assembly is put together. Typically, when bearing assembly components are inserted into the bearing housing during assembly, the bearing assembly components tend to rub against the sides of the housing. As a result of this abrasion, metal particles are often produced within the bearing assembly. Also, many bearing assemblies contain threaded holes and threaded fasteners to hold the various parts of the assembly. Pieces of metal can come loose during assembly. Because the various components of the bearing assembly are often constructed of various steel alloys, the particles thus produced typically exhibit magnetic properties. As discussed above, the presence of such particles in the x-ray tube is problematic for a variety of reasons, and may contribute to problems such as arcing, reduced bearing life, and noise generation within the x-ray device.
Finally, at least one other mechanism by which foreign matter enters the x-ray tube relates to the design of the bearing housing of the bearing assembly. In particular, the bearing housing in typical x-ray tube designs is at least partially open at the end through which the bearings and shaft are inserted. Thus, there is little to prevent foreign matter present in, or created in, the bearing assembly, from escaping into other areas of the x-ray tube. Correspondingly, the open end of the bearing housing allows foreign matter present in other parts of the x-ray tube to enter the bearings.
At least one attempt has been made to resolve the aforementioned problems and shortcomings by employing mesh, or a screen, intended to prevent foreign matter from escaping the bearing assembly. The approach represented by such screens and meshes is problematic however. For example, while such screens and meshes may be somewhat effective in confining foreign matter within the bearing assembly, they are generally ineffective in preventing the movement of the foreign matter about the interior of the bearing assembly. As discussed above, foreign matter present in, or generated in, the bearing assembly implicates a variety of undesirable consequences.
In particular, foreign matter in the bearing assembly may compromise the operation of the bearings by impairing the integrity of the bearing lubricant. As another example, the presence of foreign matter in the bearing assembly may contribute to increased noise levels in the x-ray device. Thus, the inability of screens, meshes, or similar approaches, to control the foreign matter present or created in the bearing assembly represents a significant limitation in this attempt to resolve the problems in the art.
Another limitation of known particle control methods and devices such as meshes, screens, and the like is that they are incapable of effectively controlling those particles located outside of the bearing assembly. In the x-ray tube environment, at least, this is a significant limitation because such particles, if not reliably controlled, may contribute to arcing of the anode and cathode.
In view of the foregoing problems, and others, it would be an advancement in the art to provide improved devices and systems for trapping and controlling foreign matter present in, or created in, an x-ray tube.